This invention relates to a method for removing undesirable components from polymer-solvent mixtures. In another aspect, this invention relates to a method of stripping undesirable components from a polymer-solvent mixture wherein vaporized solvent which is polymer free is used as the stripping vapor. In another aspect, this invention relates to removing undesirable components from polymer-solvent mixtures with dried solvent which has been obtained from a prior reaction and passing the purified polymer-solvent mixture to a subsequent reaction. In still another aspect, this invention relates to removing water from a conjugated diene-monovinyl arene copolymercyclohexane solvent mixture prior to metallation of the copolymer. In still another aspect, this invention relates to taking a hydrogenated butadiene/styrene copolymer in cyclohexane diluent obtained from a hydrogenation reaction, passing the mixture to a fractionation-stripping column where water is stripped from the copolymer solution by solvent vapors generated from cyclohexane which is free of polymer and then passing the recovered, dried polymer and solvent mixture to a metallation reactor for reaction with an alkyllithium, such as n-butyllithium. In still another aspect, this invention relates to a method for removing water and other components boiling at lower temperatures then the solvent from a polymer-solvent mixture employing dried solvent obtained from existing storage as the shipping medium to remove water and said other components and yield a substantially pure and dried polymer-solvent product as bottoms.
The removal of undesirable components from a polymer-solvent mixture is many times necessary prior to subsequent reaction of the polymer. The important qualities of the solvent for reaction are: (1) freedom from hydrogen donor impurities, (2) freedom from polar compounds, (3) solubility of polymers in solvent, also viscosities of polymer solutions, (4) good volatility for removal from polymer, (5) commercial availability at reasonable price, (6) low environmental problems, (7) effects on polymer structure during reaction, especially polymer solution, e.g., cis, trans, vinyl configurations, (8) low freezing point for process operations. When metalizing a hydrogenated butadiene/styrene copolymer with alkyllithium, for example, it is necessary to carry out the reaction in a very dry solution (less than 5 ppm water) since the alkyl lithium is destroyed by water. It is required, therefore, that a wet hydrogenated butadiene/styrene copolymer-solvent mixture be dried before metallation of the copolymer with butyllithium.
In a process such as the metallation of a butadiene/styrene copolymer, both the copolymer and solvent must be substantially free of water. Separation of the polymer and solvent would allow one to dry the solvent in a fractionation column and use separated solvent from the column as stripping vapors, however, a separation of the solvent and copolymer would further require drying the polymer. Two drying steps, as well as an additional separation step, would, therefore, be required as opposed to the invention method in which the polymer and solvent mixture is not separated and both the solvent and polymer are dried in a single step.
In the drying of polymer solutions, stripping vapors are frequently generated by reboiling part of the polymer-solvent product bottoms obtained from the stripping column which results in reboiler fouling. Since a polymer solution rapidly fouls heat transfer surfaces if boiling occurs in the reboiler, one method of preventing the fouling would be to employ a high flow-rate, high-pressure pump-around system with flask letdown to prevent the solution containing the polymer particles from boiling and thereby depositing the polymer particles on the reboiler to cause reboiler fouling. It would be preferred, however, if a low pressure heat transfer system could be used without causing reboiler fouling problems due to the economic savings of a low pressure heat transfer system. Also the heat transfer rates are much better for solvent compared to polymer solutions. This results in much smaller heat exchangers for the reboilers handling solvent only.
Accordingly, it is an object of this invention to provide a process wherein both the polymer and solvent of a polymer-solvent mixture taken from a prior reaction can be dried at the same time, recovered as a single product and passed to a subsequent reaction without the problem of reboiler fouling.
Another object of this invention is to provide a method for removing water from a polymer-solvent mixture without the use of a high flow-rate, high-pressure pump for generating stripping vapors.
Still another object of this invention is to provide an economical and time saving process for securing the absence of water in a polymer-solvent mixture which is passed to a metallation reaction.
Another object of this invention is to provide a process wherein water is removed from both a polymer and a solvent without the need for prior separation of the solvent and polymer.
Other aspects, objects and the several advantages of the invention will become apparent to one skilled in the art upon studying the drawing, specification and appended claims.